1. Field of the Invention
The present invention relates to a ZigBee device which is changed from a sleeping mode to an active mode as a low power wake-up method in a ZigBee communication system and an accompanied low power wake-up method, and more particularly, to a low power ZigBee device and a low power wake-up method for checking even an I.D. of a wake-up packet at a MAC processing unit of a wireless modem unit unlike a prior art in which both a wireless modem and a Central Processing Unit (CPU) are supplied with electric power for receiving and checking the wake-up packet in case that a ZigBee device receives the wake-up packet.
2. Description of the Related Art
A ubiquitous computing technology related to a wireless networking of a local area communication market such as a home and an office is one of the most noticed fields nowadays. A ZigBee, one of Wireless Personal Area Network (WPAN) technologies, has been standardized in IEEE 802.15.4 as a wireless network standard for a 2.4 GHz-based home automation and data characterized by low power, low cost and slow speed. Referring to the above-mentioned standard, the ZigBee uses frequency bandwidths of 2.4 GHz, 915 MHz and 868 MHz, and can have a transfer speed of 250 kbps (16 channels at 2.4 GHz ISM bandwidth), 40 kbps/20 kbps (10 channels at 915 MHz/one channel at 868 MHz) according to a frequency, and a modem method transfers data at a speed of 20 kbps to 250 kbps within a radius of 30 m as a Direct Secure Spread Spectrum (DSSS) and can construct a large-scale wireless sensor network indoors and outdoors by connecting 255 devices at maximum to one wireless network.
The ZigBee has an advantage of realizing the cheapest chipset since an ultra low power consumption is possible and a construction of a wireless receiving and transmitting circuit is simplified in comparison with other WPAN technologies such as Bluetooth or Ultra Wide Band (UWB), and thus it is a competitive local area wireless communication system at a vertical application area such as a sensor network. For instance, by adopting the ZigBee into illumination/fire sensing/air conditioning system in a building, a building manager can perform manage and control operations of a building system in remote control through a portable device not being in a maintenance office.
Meanwhile, a study of a System On a Chip (SoC) such as the WPAN, UWB and RFID has been processed. Generally, the SoC means that a system is embodied on one chip. The SoC for a wireless communication includes an RF unit, a modem unit, a memory, a CPU and a peripheral block, and thus low power consumption becomes an important requisition. To reduce power consumption of products becomes a factor to determine a market competitive power in the products adopting the SoC powered by a battery for the wireless communication.
In case of constructing an Electronic Shelf Labels (ESL) system by using the SoC for the wireless communication, each end device reduces power consumption by using a sleeping mode.
FIG. 1 is a flow diagram showing a wake-up method according to a prior art.
Firstly, if entering a sleeping mode (S130), a sleeping timer is set and power is cut-off (S140). A condition for an end device to be woken-up from the sleeping mode is a state in which it communicates with a high rank device, e.g., a coordinator, if its I.D. is received after periodically wakes-up and receives a wake-up packet; or a state in which it is woken-up by an external interruption and communicates with a high rank device (S150).
That is, if a wake-up event occurs, both a wireless modem unit and a CPU are woken-up and supplied with electric power for processing the wake-up packet in the conventional system of SoC (S160). Thereafter, the CPU controls the wireless modem to receive and process the wake-up packet (S170). At this time, if the wake-up packet is not its packet, it enters the sleeping mode again (S180), and if the wake-up packet is its packet, the CPU processes a data packet (S190).
It is very important to reduce power consumption since the SoC used in the sensor network is powered by a limited battery. In a general SoC, power consumption for operating the CPU accounts for large portion of whole power consumption.
Generally, for reducing power consumption of the SoC, a method of lowering an operational frequency and a method of cutting-off a power supply by cutting-off a voltage regulator have been mostly used. A system clock of the CPU used in the SoC uses a crystal-oscillator of several tens of MHz. The CPU operates a whole system by repeatedly performing an operation of reading/writing a data memory and a program memory, which consume large power.
Therefore, according to the conventional wake-up method of the SoC, both the CPU and the wireless modem are supplied with electric power if the wake-up event occurs not checking whether a packet is its packet or not, and thus unnecessary power consumption occurs.